Depression Storage and Evaporation

Depression Storage :

When the any form of precipitation reaches to the ground firstly it must fill all the depression before is can flow over the surface. The volume of water trapped in these depression does not contribute to the runoff so these are called depression storage.

Factors effect of Depression Storage:

• Type of Soil
• Nature of Depression 
• Condition of Surface
• Slope of Catchment 
• Past Precipitation. 
• Time

Experimentally evaluated values of depression storage on different type of soil, for Sand, Loam and Clay the values are 0.5 cm, 0.4 cm, 0.25 cm respectively. if we talk about the area depression storage is 0.15 cm for impervious area and 0.65 for pervious area.

Evaporation:

It is the process in which a liquid (water) changes to the gaseous state (vapour) at the free surface, below the boiling point through the transfer of heat energy. Evaporation is a cooling process such that the latent heat of vaporization is taken away from the water body by the escaping water molecule. 

If heat is given to water molecule it increases kinetic energy of the water molecule, As the kinetic energy of the water particle increases, they tends to leave the surface of water body and some water particle tends to come in water body from air. If the net particle is going outward of body then it is called evaporation and if the net particle is going inward of water body then it is called condensation.

Factors affects on Evaporation:

• Temperature 
• wind Speed
• Atmospheric Pressure
• Quality of Water
• Size of Water Body
• Humidity

Temperature :

The rate of evaporation increases with an increase in water temperature. Regarding the air temperature, increase in evaporation rate with increase temperature, but high correlation between evaporation rate and air does not exist. It is possible that evaporation will be different in a different month if the mean monthly temperature is same in all month.

Wind Speed : 

If the wind velocity is large enough to remove all the evaporated water vapour, any further increases in wind velocity does not influence the evaporation. Thus, the of evaporation increases with the speed up to a critical speed beyond which any further increases in the wind speed has no influence on the evaporation rate. For large water bodies high speed turbulent wind are needed to cause maximum rate of evaporation.

Atmospheric Pressure :

A decrease in the atmospheric pressure increase the evaporation as seen in high altitude where the atmospheric pressure reduces.

Quality of Water :

If a salt is dissolved in water, the vapour pressure of the solution is less that that of pure water and hence causes the reduction in the rate of evaporation. Sea water is more saline so they have 2-3 % less evaporation.

Size of Water Body :

For a large water body surface the evaporation rate is highly influenced by the depth of water.

The increase in depth of water body reduces the rate of evaporation from the surface of water body in summer season and in winter season evaporation will increases with the increase of the depth of water body.

Humidity :

If the humidity of the atmosphere is more, the evaporation will be less and vise versa.

Factors affects the evaporation of soil moisture :

• Soil Moisture Content
• Soil Capillary Characteristics 
• Depth of Water Table
• Colour of Soil  

Soil Moisture Content :

The evaporation from saturated soil is more that the evaporation of same surface water body. 

Soil Capillary Characteristics :

Capillary supply the water content to the surface so they increases the evaporation.

Depth of Water Table :

Soil evaporation is maximum when the water table is at the surface and decreases quiet rapidly as the water table goes down.

Colour of Soil :

Soil colour also affects the evaporation rate, A darker soil absorbs more heat and increases the temperature, so increases the evaporation rate.